More than 95% of patients with chronic myelogenous leukemia (CML) possess the Philadelphia chromosome (Ph.sup.1), an abnormal chromosome present in leukemia cells that originates from the reciprocal translocation between chromosomes 9 and 22. This chromosomal exchange joins the 5' two-thirds of the BCR gene remaining on chromosome 22 to a large portion of the ABL gene translocated from chromosome 9. The BCR gene is described in more detail in Groffen et al., Cell 36:93 (1984), while the ABL gene is described in Heisterkamp et al., Nature 306:239 (1983) and in Gale & Canaani, PNAS (U.S.A.) 81:5648 (1984).
The fused genes generate a hybrid mRNA with a continuous open reading that encodes a BCR-ABL protein of more than 2,000 amino acids. Two types of BCR-ABL transcripts have been detected which differ by a small coding exon originally termed the 3' bcr exon within the breakpoint cluster region of the BCR gene. These two transcripts encode two BCR-ABL proteins of 2023 and 1999 amino acids, depending upon whether the 3' bcr exon is present in the hybrid mRNA. These proteins migrate in sodium dodecyl sulfate (SDS) polyacrylamide gels with an estimated size of 210,000 daltons and are termed P210 BCR-ABL.
The Ph.sup.1 is also present in some patients with acute lymphocytic leukemia (ALL) and acute myelogenous leukemia (AML). Ph.sup.1 -positive ALL patients express either the typical P210 BCR-ABL protein or a smaller protein termed P185 BCR-ABL. The latter results from a more 5' break in the BCR gene within a large intron between exons 1 and 2.
Both the CML and ALL forms of the BCR-ABL protein possess an activated tyrosine kinase activity. The assay for the BCR-ABL tyrosine kinase activity is widely used as a means to detect the BCR-ABL protein in cell lines derived from blast crisis CML patients and is described in Maxwell et al, Cancer Res. 47:1731 (1987). The BCR-ABL protein kinase assay has also been used to detect P210 BCR-ABL in uncultured cells from patients in blast crisis as reported in Maxwell et al., supra. Although the BCR-ABL protein can be routinely detected in blood cells from blast crisis CML patients by assaying for its activated tyrosine kinase activity, detection of P210 BCR-ABL in early-stage CML patients (chronic phase) has not yet been possible according to Maxwell et al. Attempts to detect the BCR-ABL protein in chronic-phase patients by this assay have been hindered by large numbers of mature cells in blood and bone marrow samples in these patients.
According to published reports, the patient samples contain high concentrations of degradative enzymes. The tyrosine kinase activity, which is detected in blast crisis cell samples that contain more than 50% blast cells, is rarely detected in samples from chronic-and accelerated-phase patients. The extracts of mature granulocytes, either from normal or CML patients, rapidly destroy P210 kinase activity from extracts of Ph.sup.1 -positive cultured cells according to experiment described in Maxwell et al., supra.
The degradative factors in mature white blood cell populations must be inhibited before the tyrosine kinase assay of BCR-ABL proteins can be used to monitor patients for the presence of leukemic cell clones. This inhibition has not yet been possible to achieve.
Thus, a need exists for an assay that can detect and quantitate Ph.sup.1 -positive leukemic cell proteins. Such assays would be useful for diagnosing and monitoring patients in the early phases of CML and other types of leukemia. The present invention satisfies this need and provides related advantages as well.